1. Field of the Invention
This invention relates to the recovery and purification of ethylene and optionally propylene from a multi-component gas mixture which contains components that are both lighter and heavier than the olefins that are to be recovered and purified, and more particularly relates to the recovery of ethylene and optionally propylene from the cooled effluent of a steam cracking furnace by a process employing an ethylene distributor column.
2. Discussion of the Prior Art
The method of the present invention can be employed generally for recovering and purifying ethylene and optionally propylene but is described herein in the context of recovering and purifying ethylene and optionally propylene from the effluent of a steam cracking furnace. In a typical ethylene manufacturing plant, hydrocarbon feeds are vaporized if necessary, preheated, mixed with steam, and directed to a steam cracking furnace. Many different kinds of hydrocarbon feeds can be used, including ethane, propane, butane, naphthas, distillates, and gas oils. Mixtures of hydrocarbons can also be used, but because the optimum furnace conditions for each feed type vary, it is preferable that different hydrocarbons be segregated and cracked in different furnaces.
In a steam cracking furnace the relatively low-pressure hydrocarbon and steam mixture is subjected to high temperatures which convert the hydrocarbon into a furnace effluent gas mixture, typically comprising ethylene, methane, hydrogen and unconverted feed, as well as some hydrocarbons heavier than the feed. The hot furnace effluent gas is cooled by raising high pressure steam and also typically by direct contact with circulated cooled quench oil and/or circulated cooled water. These cooling steps typically condense and at least partially remove relatively heavy hydrocarbons, typically in the naphtha range and heavier.
The uncondensed cooled effluent gas is then directed to a compressor section in which the gas is compressed in one or more stages (typically 3-5 stages) to an elevated pressure. The effluent from each stage is typically cooled against an ambient temperature medium and any condensed liquids removed before entering the subsequent compression stage. Acid gases such as H2S and CO2 are generally removed after one of these stages of compression, for example through the use of a caustic contacting tower or an amine scrubbing system. Once compressed, scrubbed and dried, the furnace effluent gas enters the separation section.
A typical ethylene plant separation section employs a number of distillation towers for the purpose of recovering ethylene from the furnace effluent gas and purifying it sufficiently for use in downstream processes, such as the manufacture of polyethylene. A number of alternatives exist for the design of the ethylene separation section. Typically ethylene separation designs will employ at least a deethanizer tower which has the purpose of separating C2 and C3 components (that is, ethylene and ethane from propylene and propane, respectively), a demethanizer tower for separating C2 components from any components lighter than the C2s, and a C2 splitter for the final separation of ethylene from ethane.
The use of distillation to purify products from olefins plants is well known in the art. Conventional distillation schemes typically have utilized “sharp-split” distillation, wherein each distillation column is used to make a sharp separation between adjacent components of a homologous series. In a sharp-split distillation sequence, each component leaves the distillation column in a single product stream, either as overheads or bottoms. An inherent inefficiency in sharp-split distillation can be observed by considering the number of phase changes necessary to produce a recoverable hydrocarbon component. For example, a hydrocarbon gas feed typically containing C1+ hydrocarbons, such as ethylene, is first condensed in a demethanizer, then revaporized in a deethanizer, and is finally condensed again as a liquid product from a C2 splitter. A total of three complete phase changes must be accomplished for all the ethylene. The same number of phase changes applies to ethane and propylene.
The energy required to recovery and purify a hydrocarbon component such as ethylene can be reduced by utilizing a refinement upon conventional, sharp-split distillation. Such a refinement is known as distributed distillation. Such schemes require less energy to operate than conventional sharp-split schemes. In distributed distillation schemes, sharp cuts are not necessarily made between components. Instead, one or more of the components is “distributed” between the top and bottom of one or more distillation columns. This results in energy savings in part because the total number of phase changes necessary to produce ethylene product is reduced compared with a sharp split flowsheet, and the thermodynamic efficiency of the process is therefore improved. In addition, distributed distillation provides additional degrees of freedom for energy optimization—namely, the distribution ratio of the distributing components in each column.
The present invention also relates to the use of an ethylene distributor column for the recovery and purification of ethylene. For the purpose of this invention, an ethylene distributor column is one in which a sharp split is made between components lighter than ethylene and components heavier than ethylene. Therefore the ethylene distributor overhead stream contains ethylene and any components lighter than ethylene that enter the ethylene distributor. In particular, the ethylene distributor overheads contain a sufficiently low concentration of ethane that no further ethane/ethylene separation is needed in order to produce a purified ethylene product from this stream. The ethylene distributor bottoms stream contains ethylene and any components heavier than ethylene that enter the ethylene distributor.
Additional energy savings can be gained by thermally coupling (also called recycle-coupling) columns such that all or at least part of the stripping vapor or reflux liquid of a column is provided by a vapor or liquid side-draw from a downstream tower. Furthermore, the use of a mixed refrigerant system to provide the required coldest level refrigeration requirements would further reduce the energy requirement of such a separation system. Examples of a completely thermally coupled distributed distillation system have been disclosed in Manley et al., U.S. Pat. Nos. 5,675,054 and 5,746,066, which disclose the use of an ethylene distributor column and a mixed refrigeration system in a complete thermally coupled distributed distillation system. Both patents disclose thermally coupled embodiments for ethylene separation, including an embodiment that recites a front-end depropanizer ethylene recovery and purification process that utilizes full thermal coupling of the C2s distributor, deethanizer, demethanizer, ethylene distributor, and C2 splitter columns. The thermal coupling of the columns is integral to the claimed energy efficiency of this prior art process.
All of the columns recited in Manley's embodiments operate at substantially the same pressure, with any differences in pressure due to typical hydraulic pressure drops through the columns, exchangers, and piping. Substantial differences in pressure between the columns would require vapor compression or liquid pumping between columns. Manley recites that such a fully-coupled distributed distillation system has lower energy requirements than systems that are not thermally coupled. Conventional wisdom also suggests that such an arrangement, being fully thermally coupled, would be more energy efficient than a scheme that has no couples or is only partially thermally coupled.
Furthermore, neither of these patents discloses a separation of hydrogen and methane intermediate between the ethylene distributor tower and the demethanizer tower, which would be beneficial for increasing the recovery of hydrogen to salable product. It could also be beneficial from both the energy and operability standpoint to replace some of the thermal coupling with, for example, a standard reboiler to provide stripping vapor to the ethylene distributor.
However, while a completely thermally coupled arrangement would require the lowest overall heating and cooling duty, it does not necessarily represent the lowest energy solution when the refrigeration compression energy required to service the sub-ambient duties is considered. By considering these additional design aspects we have discovered, surprisingly, a partially coupled scheme that is actually more energy efficient than the fully-coupled scheme described by Manley. In particular, two of the thermal couples taught by Manley et al., specifically the thermal couple between the C2 distributor and deethanizer columns and the thermal couple between the ethylene distributor and the deethanizer or C2 splitter, actually increase the energy requirement for the process when implemented in a conventional cracker with conventional vapor recompression refrigeration systems. The distillation system of this invention, therefore, does not include these couples and represents an unexpected improvement in energy savings as compared to Manley et al. In addition, it has been found that removing these two thermal couples allows the deethanizer and/or C2 splitter to be operated at a lower, more optimal pressure than the rest of the distillation sequence. The full thermal coupling recited by Manley et al., on the other hand, requires that all columns be operated at roughly the same pressure, or utilize energy intensive vapor compression between columns. A partially coupled scheme can also be an improvement from any operability standpoint relative to a fully thermally coupled scheme.
Another disclosure of the use of an ethylene distributor is in Kuechler et al., U.S. Pat. No. 6,212,905, which teaches a process in which a secondary ethylene product stream is recovered from a mixed gas stream at a temperature higher than −55° F. The patent does not disclose a separation of ethylene from components lighter than ethylene, and thus the ethylene product stream can have significant levels of components lighter than ethylene. In the case of steam cracking, therefore, this secondary ethylene product would contain undesirably high levels of both methane and hydrogen, rendering it unfit for use in most ethylene conversion processes, such as the manufacture of polyethylene.
Surprisingly, we have found that making a rough separation of methane and hydrogen downstream of the ethylene distributor and upstream of the hydrogen recovery and purification section of the plant significantly increases the hydrogen recovery of the process with only a small increase in energy levels. In contrast to standard distributed distillation systems, a hydrogen depleted gas is expanded and used for refrigeration, so less hydrogen is degraded from chemical to fuel value. This overcomes one of the disadvantages of prior art ethylene recovery systems based on an ethylene distributor, namely, low hydrogen recovery. We have further found that the methane rich gas from the aforesaid rough separation can be expanded and chilled to provide a cooling duty to the overall process.